Examples of high pressure fuel injection systems are shown in U.S. Pat. No. 5,020,500 issued to Kelly on Jun. 4, 1991, and U.S. Pat. No. 5,191,867 issued to Glassey et al. on Mar. 9, 1993. Engines equipped with high pressure fuel injection systems have an optimal volumetric injection rate. For diesel-cycle engines, this optimal injection rate typically has a gradual rise, a period of stabilization, followed by a sharp drop. Means of producing this characteristic profile are commonly referred to as rate shaping means or devices because they are used to shape the volumetric rate of fuel injection into an engine combustion chamber. The gradual rise followed by a sharp drop in fuel injection has the specific benefits of minimizing both particulate emissions and noise from combustion.
Fuel injector nozzles typically include a housing with an elongated cavity or void along a first axis. The cavity has a first end portion or injection chamber and a second end portion or spring chamber with a guide passage disposed therebetween. At least one injection orifice fluidly connects the injection chamber of the cavity with an atmosphere (e.g., engine combustion chamber) external to the fuel injector. A needle check is slidably disposed within the cavity for translation between a first position in which a seat portion of the needle check seats against a first end portion of the cavity in the injection chamber, thereby blocking the passage of fuel through the injection orifice, and a second position wherein the needle is spaced from the first end portion and does not block the injection orifice.
The fuel injector of Glassey et al. is configured largely as described above. A guide passage of the cavity is in slidable contact with the guide portion of the needle check. Pressurized fuel directed to the injection chamber of the cavity overcomes a biasing spring to move the check away from the first end portion. Glassey does not teach the provision of any rate shaping means in the nozzle other than the small amount of rate shaping inherently provided by a hydraulically-actuated injector.
The fuel injector nozzle disclosed by Kelly is generally similar to that of Glassey et al., except that it includes a second guide passage near the first end portion with which the needle check is in slidable contact near its seat portion. This second guide passage is disposed between a location where high pressure fuel is introduced to the injection chamber and the first end portion of the cavity with the injection orifice. For fuel to reach the injection orifice, it must first seep through a very restrictive flow area between the check and the second guide passage to develop sufficient pressure between the first end portion of the cavity and the check so that the force against the needle check can overcome a resisting spring. Once the check is partially lifted to a predetermined height, an enlarged passage with a much larger flow area between the needle and the second guide passage is opened, providing more direct fluid communication between the source of high pressure fuel and the first end portion of the cavity. At that point of operation, the volumetric rate of fuel passing through the orifices increases. It is the selective variance of the rate of fuel reaching the orifices which is characterized as rate shaping. However, the specific configuration of the injection chamber and guide passages disclosed by Kelly, particularly the second guide passage, is relatively difficult to fabricate with the degree of precision necessary with tooling suited for high volume production.
It is desired to provide a fuel injector nozzle having rate shaping characteristics provided by initially restricting the flow of fuel between a guide passage of a check cavity and a guide portion of the check until the check reaches a predetermined lift height avoiding the use of a second guide passage near the first end portion.